In recent years, energy conservation has been regarded as a significant technical objective for copy machines, printers, and fax machines and significant reduction of the amount of heat used in fixing units has been desired. Under such trends, there is a growing demand for low fixing temperature toners that can be fixed with low energy.
With a growing demand for these apparatuses worldwide, there has also been an increasing demand for apparatuses that can stably produce high-quality images in various operating environments, in particular, environments of various different temperature and humidity. Under such trends, the toners used in such apparatuses are desired to exhibit charge properties unaffected by the temperature and humidity and have high durability that causes less image deterioration even after a large number of copies and printouts are made.
An example of a typical method for improving the low temperature fixability of toners is to decrease the glass transition temperature (Tg) of binder resins in the toners. However, merely decreasing the Tg of binder resins impairs the thermal storage resistance of the toner. It has been considered difficult to achieve both the low temperature fixability and the thermal storage resistance.
In order to achieve the low temperature fixability and the thermal storage resistance at the same time, feasibility of using a crystalline resin that has a highly sharp melting characteristic as the binder resin has been investigated.
Amorphous resins generally used as binder resins in toners do not have endothermic peaks in differential scanning calorimetry (DSC) measurement. Binder resins that contain crystalline resins exhibit endothermic peaks in DSC measurement. The peak temperature of the endothermic peak is the melting point of the crystalline resin.
Crystalline polyester resins have a structure in which polymer chains are regularly aligned, do not readily soften in the temperature region below the melting point, and have a property of rapidly melting and undergoing a decrease in viscosity at and after the melting point. Because of these properties, crystalline polyester resins have drawn much attention in recent years and studies have been actively made on use of crystalline polyester resins as a material of toners.
PTL 1 proposes a toner obtained by dispersing in a liquid or supercritical carbon dioxide a solution of a resin constituted by an amorphous segment and a crystalline segment containing an aliphatic polyester (i.e., crystalline polyester) as an essential component in an organic solvent so as to form resin particles containing the resin and the organic solvent, and then removing the organic solvent and carbon dioxide.
PTL 2 proposes a toner obtained by dispersing in a liquid or supercritical carbon dioxide fine particles containing a resin that contains a vinyl monomer having a crystalline polyester chain as an essential constitutional unit to prepare a dispersion, and dispersing in this dispersion a solution of a resin that will serve as a binder resin in an organic solvent so as to form resin particles having the fine particles fixed to the surfaces, and then removing the organic solvent and carbon dioxide.
Compared to typical toners that use amorphous resins as binder resins, these toners exhibit superior low temperature fixability due to the sharp melting characteristic of the crystalline polyester. However, studies conducted by the inventors of the present application have shown that these toners do not necessarily have sufficient charge properties and have a tendency of not stably retaining the amount of charge after triboelectric charging. When a developer that contains such a toner is left unstirred, scattering of the toner to non-image portions and image defects readily occur in the subsequent development step and high-quality images are not always obtained.
The cause thereof is presumably that crystalline polyesters have a volume resistivity lower than typical amorphous resins and leakage of charge readily occur when the crystalline polyester contained in the toner is exposed in particle surfaces.
Another possible approach for improving the environmental stability of the toner is to cover the toner particle surfaces with a hydrophobic material. Organic polysiloxanes are known to have a low surface tension. Thus, introducing an organic polysiloxane structure into surface portions of toner particles may produce a toner that has a charge property unaffected by the ambient humidity.
PTL 3 proposes a toner obtained by dispersing an organic solvent solution of a resin containing a crystalline polyester into liquid or supercritical carbon dioxide containing dispersed fine particles containing a resin that contains a vinyl monomer (silicone-containing vinyl monomer) having an organic polysiloxane structure as an essential constitutional unit so as to form resin particles having the fine particles fixed to the surfaces, and then removing the organic solvent and carbon dioxide.
The toner obtained in accordance with this disclosure was evaluated in terms of the fixability. It was found that compared to other toners having similar melt viscosity characteristics, this toner had a tendency to readily separate on rubbing of the surface of the fixed image.
The cause for this is presumably that the ratio of the vinyl monomer having the organic polysiloxane structure contained in the resin is excessively large and this made the toner susceptible to the influence of surface tension and decreased the adhesion between the fused toner and paper.
This toner was also evaluated in terms of durability. It was found that the toner had a tendency to cause development banding as the evaluation cycle was repeated.
The cause for this is presumably that since an organic polysiloxane usually has a glass transition temperature (Tg) lower than room temperature, the degree of polymerization (molecular weight) of the siloxane segment in the vinyl monomer became excessively high, and that since the ratio of the vinyl monomer having the organic polysiloxane structure contained in the resin was excessively large, the hardness of the toner surface became insufficient and fusing onto a regulation member occurred.
As discussed above, toners that contain crystalline polyesters have to be improved to achieve sufficient low temperature fixability and stable charge property. Moreover, in order to improve the environmental stability by introduction of organic polysiloxane structures, the stability and durability of the fixed images have to be improved.
It is desirable to provide a toner that address the issues described above. It is desirable to provide a toner that has good low-temperature fixability, charge stability, environmental stability, and durability, and is capable of stably producing high-quality images for a long time.